Cerebral palsy (CP) is a heterogeneous disorder in which movement and posture are affected. Increased excitation of the central nervous system leads to neural symptoms, which can cause spasticity and muscle weakness. These neural abnormalities result in secondary CP-related mechanical adaptations of muscles and tendons, which can lead to muscle contracture, joint deformities and pain. Therapeutic interventions are therefore essential to treat CP-induced abnormalities. Passive stretching in particular is a popular treatment method in clinical practice. However, due to a lack of scientific evidence, clinicians often have to make assumptions about the mechanical adaptability of muscles and tendons. Currently, the mechanical properties of muscles and tendons in children with CP and their adaptability are not well understood, which makes it difficult to implement evidence-based practice in clinical settings. Therefore, the overall purpose of this research was to examine the mechanical properties of the medial gastrocnemius muscle and Achilles tendon in children with spastic CP, and the adaptations of the muscle and tendon to acute and long-term passive stretching. The first experimental Chapter (3) was carried out in healthy adults, to assess the agreement between two methods of deriving Achilles tendon stiffness (i) active contraction of the triceps surae muscles to elongate the Achilles tendon, or (ii) passive rotation of the ankle joint. Taking into consideration the tendon’s viscoelastic response, the effects of strain-rate on Achilles tendon stiffness were also described. Results revealed that tendon stiffness measured using the “active method” was 6% greater than the “passive method”. There was also a significant increase in Achilles tendon stiffness in response to increased strain-rate. As the more commonly used active method is problematic to be used in children with CP, due to muscle weakness and excessive co-contraction, the passive method of deriving tendon stiffness was used in subsequent experimental studies. In experimental Chapter 4, differences in the mechanical properties of the Achilles tendon and triceps surae muscles between children with CP and their typically developing (TD) peers, were investigated. The results revealed that estimates of triceps surae muscle stiffness were significantly greater in children with CP compared to TD children. The results also showed that despite a smaller tendon cross-sectional area in children with CP, Achilles tendon stiffness was not different between groups. In addition, children with CP had a steeper tendon stiffness-strain-rate relationship compared to TD children. These results have significant clinical implications regarding the diagnosis of spasticity using the current clinical methods. Experimental Chapters 5 and 6 examined the muscle’s and tendon’s response to stretch. Passive stretching, implemented by a clinician or by the children themselves, is a commonly used intervention for children with CP with the aim of inducing structural alterations in muscles and tendons to improve function. In order for these alterations to take place, elongation of the muscle and fascicles would presumably need to occur with acute stretching. To date, this assumption has not been tested. Thus, the purpose of Chapter 5 was to investigate the medial gastrocnemius and muscle fascicle response to acute stretching, using two commonly used stretch techniques. Results of this study revealed that 100 s of stretching caused a transient increase in tendon (1.0 cm), muscle (0.8 cm) and fascicle lengths (0.6 cm). This effect was independent of stretch technique. These results provide evidence that the muscle and fascicles are capable of elongating in response to stretch in children with spastic CP. They provide a basis for the hypothesis that the spastic muscle may be able to adapt in response to long-term stretching. Thus, the purpose of the final experimental Chapter (6) was to assess the effects of a six week passive stretching intervention (four days per week, 15 minutes per day) on muscle and tendon properties, and gait parameters in children with CP. Results revealed there was a significant reduction in joint stiffness in the experimental group following six weeks of stretching. This was accompanied by a reduction in muscle stiffness, but with no alterations in Achilles tendon stiffness. Additionally, there were no positive effects of passive stretching on gait parameters. Together, the results of the present series of investigations demonstrates how fundamental knowledge of muscle and tendon mechanics in children with spastic CP, can be implemented to support evidence-based clinical practice.